The field of the invention relates to ignition timing systems with knock control. In particular, the invention relates to ignition systems having both minimum spark for best torque (MBT) ignition control and knock control accomplished on an individual cylinder basis.
Optimal torque output is achieved when ignition timing of an engine is set at MBT. The ignition timing of a particular model of motor vehicle is typically set or calibrated at a predefined spark advance before top dead center (TDC) such that the average of all such vehicles, when new, is near MBT. This general approach has been found to be less than optimal for two basic reasons. First, vehicle calibrators are forced to set ignition timing at a value appreciably less than MBT to avoid knocking under certain operating conditions. Second, variations among engines, subsequent maintenance, environmental conditions, and aging often result in an actual MBT which is different from the initial spark advance calibration or reference MBT.
Knock control systems are known wherein ignition timing is retarded a predetermined increment upon each and every detection of knock. When knock does not occur, ignition timing is typically advanced a smaller increment to allegedly hunt for an optimal ignition timing. A disadvantage of such systems is that each occurrence of a knock results in ignition retarding. This has been recognized to be a less than optimal solution because optimal torque output is typically achieved with occasional knocking referred to as trace knock. Thus, these systems tend to excessively retard ignition timing resulting in less efficient engine operation. Another disadvantage of these systems is that in the absence of knock, ignition timing is only advanced back to the original reference value. Stated another way, these systems do not determine and achieve an actual MBT value.
Recognizing the above disadvantage of retarding upon each detection of knock, a number of approaches utilize a frequency of knock detection. More specifically, a predetermined time interval is generated by counting a predetermined number of engine cycles, such as 1,000 cycles. The number of knock detections during this predetermined number of engine cycles is then counted and compared to a reference value. Examples of these approaches are found in U.S. Pat. No. 4,120,272 issued to Douaud et al, U.S. Pat. No. 4,002,155 issued to Harned et al, U.S. Pat. Nos. 4,261,313 and 4,274,379 issued to Iwata et al. The inventor herein has recognized a disadvantage of slow response time inherent in the above approaches. More specifically, a timing correction cannot be made until the predetermined number of engine cycles is counted. Thus, under severe knocking conditions, excessive time may elapse before a knock correction is made resulting in serious engine damage. Another disadvantage of the above approaches, is that ignition timing is only advanced back to the best guess or reference value of MBT. Actual MBT control is not disclosed.
U.S. Pat. No. 4,466,405 issued to Hattori et al discloses an ignition timing system having both knock control and MBT control. Like the approaches described above, the '405 patent discloses a frequency of knock detection by counting the occurrences of knock during a predetermined number of engine cycles. Knock trim values are read into a random access memory (RAM) as a function of speed and load. Independently generated MBT values are also read into the same RAM. The inventor herein has recognized numerous disadvantages in the disclosure of '405 patent. As in the case of the approaches described above, knock corrections cannot be made until a predetermined number of engine cycles are counted. The resulting slow response time may cause engine damage under some operating conditions. A further disadvantage, is that MBT and knock control cannot be concurrently conducted. Accordingly, approaches of this nature may tend to hunt, or oscillate, around the timing reference.